Route gh host qualification through runner options instead of argv sniff

Move GHES/GH_HOST resolution from parsing --hostname/--repo out of gh argv to an explicit options.host passed through ghExecFileAsync, since SSH-backed repos spawn gh with no cwd and argv sniffing couldn't reliably detect the target host. The runner now injects --hostname and qualifies --repo/-R at spawn time from options.host, and rate-limit scoping/guards use the same explicit host instead of inferring it. Also adds a shared githubRepoIdentityKey helper to keep cache/store keys consistent with the new host-aware repository identity.

src/main/github/client-issue-source.test.ts

@@ -44,7 +44,10 @@ vi.mock('./gh-utils', async () => {
 vi.mock('./rate-limit', () => ({
   rateLimitGuard: rateLimitGuardMock,
   noteRateLimitSpend: noteRateLimitSpendMock,
-  getRateLimit: vi.fn(async () => ({ ok: false, error: 'not probed in tests' }))
+  getRateLimit: vi.fn(async () => ({ ok: false, error: 'not probed in tests' })),
+  repositoryRateLimitGuard: vi.fn(() => ({ blocked: false })),
+  noteRepositoryRateLimitSpend: vi.fn(),
+  spendsSharedGitHubComQuota: () => true
 }))
 
 import { countWorkItems, getWorkItem, listWorkItems, _resetOwnerRepoCache } from './client'